Non-cryogenic bulk oxygen separation systems, for example, organic polymer membrane systems, have been used to separate selected gases from air and other gas mixtures. Air is a mixture of gases which may contain varing amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. An entirely different type of membrane, however, can be made from certain inorganic oxides. These solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure.
Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. The most obvious difficulty is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 500.degree. C., generally in the 600.degree. C. to 900.degree. C. range. This limitation remains despite much research to find materials that work well at lower temperatures. Solid electrolyte ionic conductor technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art. The elevated temperatures of operation, however, make ion transport processes intrinsically well suited for intergration with high temperature processes such as vapor-based, gas-based, or combined power cycles.
Recent developments have produced solid oxides which have the ability to conduct oxygen ions at elevated temperatures if a chemical driving potential is applied. These pressure-driven ionic conductor materials may be used as membranes for the extraction of oxygen from oxygen-containing gas streams if a sufficient partial oxygen pressure ratio is applied to provide the chemical driving potential. Since the selectivity of these materials for oxygen is infinite and oxygen fluxes several orders of magnitude higher than for polymeric membranes can be obtained, attractive opportunities are created for the production of oxygen as well as for oxygen-requiring oxidation processes, especially with applications that involve elevated temperatures. A prominent example is gas turbine cycles which typically process a significant amount of excess air to keep the turbine inlet temperature within the capabilities of available materials and therefore make available excess oxygen for recovery as a coproduct.
Some of the key problems that have to be addressed in the design of ion transport membrane systems and their integration into a high temperature cycle such as a gas turbine involve maximizing driving forces for ion transport, minimizing gaseous diffusion resistance, avoiding excessive stresses from thermal and compositional expansion and contraction and sealing the ion transport elements within the ion transport apparatus. The latter problem is aggravated by the fact that the ion transport membrane operating temperature is in the range from 500.degree. C. to 1100.degree. C.
Advances in the state of the art of air separation using solid electrolyte ionic conductors have been presented in the technical literature. For example, Mazanec et al., U.S. Pat. No. 5,306,411, entitled Solid Multi-Component Membranes, Electrochemical Reactor Components, Electrochemical Reactors and Use of Membranes, Reactor Components, and Reactor for Oxidation Reactions, relates to electrochemical reactors for reacting an oxygen-containing gas with an oxygen-consuming gas and describes a shell and tube reactor with the oxygen-containing gas flowing on one side of the solid electrolytic membrane and the oxygen-consuming gas on the other. Mazanec et al., however, does not address issues related to integrating such systems with oxygen production from gas turbine cycles, heat management to maintain membrane surfaces at the desired uniform temperatures, flow dynamics to achieve effective mass transfer, or the need for balancing reaction kinetics with oxygen ion conductivity to maintain the appropriate oxygen partial pressure for materials stability.
Kang et al., U.S. Pat. No. 5,565,017, entitled High Temperature Oxygen Production with Steam and Power Generation, relates to a system integrating an ion transport membrane with a gas turbine to recover energy from the retentate gas stream after it is heated and steam is added. The injection of steam or water into the ion transport nonpermeate stream compensates for the loss of the oxygen mass from the turbine feed gas stream.
Kang et al., U.S. Pat. No. 5,562,754, entitled Production of Oxygen By Ion Transport Membranes with Steam Utilization, discloses a system integrating an ion transport membrane with a gas turbine to recover energy from the retentate gas stream after it is heated. Steam is added as a sweep gas on the permeate side to enhance oxygen recovery. A stream containing a mixture of oxygen and steam is produced on the permeate side which can be withdrawn as a product.
Kang et al., U.S. Pat. No. 5,516,359, entitled Integrated High Temperature Method for Oxygen Production, describes heating a compressed air feed gas stream to the appropriate ion transport operating temperature by a first combustor which, in one embodiment, is inserted between the compressor discharge and the ion transport separator. Subsequently, the retentate gas stream from the ion transport separator is heated to turbine inlet temperature by a second combustor. The inlet temperatures to the turbine and the ion transport separator are independently controlled by adjusting the fuel rates to the two combustors. In an alternate configuration, a heat exchanger is placed between the two combustors, which are both located downstream from the ion transport separator, and the air feed gas stream to the ion transport separator is heated to the correct temperature by this heat exchanger.
The method disclosed in the Kang et al. '359 patent has the disadvantage that the feed gas stream to the ion transport separator contains products of combustion which dilute the ion transport separator feed gas stream, reduce the oxygen driving force, and could act as an adverse contaminant to some mixed conductor materials. Because some of the oxygen contained in the feed air is consumed in the first combustor, the oxygen driving force is further reduced. The alternative configuration has the disadvantage of requiring an additional, potentially expensive, heat exchanger. Either method could also have a problem maintaining stable combustion in the first combustor since the fuel-to-air equivalence ratio will be low, especially where the heat contained in the turbine exhaust is regeneratively recovered by the feed air gas stream and the heat duty of the first combustor is small.
Hegarty, U.S. Pat. No. 4,545,787, entitled Process for Producing By-Product Oxygen from Turbine Power Generation, relates to a process for generating net power using a combustion turbine, accompanied by the recovery of by-product oxygen-enriched gas. Air is compressed and heated, at least a portion of the air is combusted and a portion of the oxygen is removed from the air or combustion effluent using an air separator. The oxygen lean combustion effluent is expanded through a turbine to produce power. In an alternative embodiment, the effluent from the turbine is used to produce steam to generate additional power. In this process, the type of fuel is generally limited to "clean" fuels such as natural gas, oils, or synthesis gas.
Chen, U.S. Pat. No. 5,035,727, entitled Oxygen Extraction from Externally Fired Gas Turbines, relates to a process for recovering high purity oxygen from an externally fired power generating gas turbine cycle. While this process is similar to the Hegarty '787 patent, Chen differs in the use of an externally fired gas turbine so that other types of fuels such as coal or biomass may be used.
Chen et al., U.S. Pat. No. 5,174,866, entitled Oxygen Recovery from Turbine Exhaust Using Solid Electrolyte Membrane, and Chen et al., U.S. Pat. No. 5,118,395, entitled Oxygen Recovery from Turbine Exhaust Using Solid Electrolyte Membrane, both relate to processes for extracting high purity oxygen from gas turbine exhaust streams by passing the gas turbine exhaust over an oxygen ion conducting membrane. In these processes, the oxygen separator employing an oxygen ion conducting membrane is placed downstream of some or all stages of the gas turbine, instead of upstream as in earlier patents. An electrically-driven ion transport unit is proposed when the turbine exhaust pressure is low. The exhaust stream from the oxygen separator is optionally expanded through a gas turbine stage.